Introduction

The osmometer is a device used in clinical laboratories for measuring the concentration of particles in a solution, known as the osmolar concentration. This quantity can be expressed as osmolality (in units of mmol/kg) or osmolarity (in units of mmol/L). Clinical laboratories usually measure osmolality. Osmolality is considered more precise because weight is temperature-independent.[1] In laboratory analysis, a dissolved substance is referred to as a solute, and the substance in which the solute is dissolved is referred to as a solvent. A solute dissolved into a solvent creates a solution. The unit for osmolar concentration or osmolality is milliosmole (abbreviated mOsm or mOsmol). For nonelectrolytes, 1 mmol equals 1 mOsm, while for electrolytes, the number of particles in a solution depends on the electrolyte's dissociation.[2][3]

Four colligative properties of a solvent change if a solute is dissolved into a solvent. These properties are:

• Osmotic pressure
• Vapor pressure
• Boiling point
• Freezing point

These properties are tied to osmolality and depend on the solution's number of solute particles. Dissolving a solute into a solvent generally increases the osmotic pressure and boiling point and decreases the vapor pressure and freezing point of a solution. While any of the four colligative properties could be used to determine the osmolality of a solution, technical limitations restrict the commercial measurement of osmolality to freezing point, vapor pressure, and membrane osmometers. The freezing point depression method is the most common in clinical laboratories because it is the most accurate and simple method.[4] 

The general principle of freezing point depression osmometers involves the relationship between the number of moles of dissolved solute in a solution and the change in freezing point. For example, one mole of a dissolved solute reduces the freezing point of water by approximately 1.86 degrees Celsius (~35.35 degrees Fahrenheit).[5] Therefore, freezing and thawing a solution and comparing the relative change in freezing point in reference to a pure solvent allows for the determination of the approximate number of moles of dissolved solute in a sample. In clinical laboratory analysis, most samples are in water-based aqueous solutions, and the reference for solutions is generally pure water.

The setup of a freezing point depression osmometer includes a temperature-controlled bath to allow for sub-freezing temperatures, a thermistor probe connected to a Wheatstone bridge circuit to measure the temperature of a clinical sample, and a thermistor readout circuit, which represents a combination of a galvanometer and a potentiometer.[5][6] While many osmometers are individual tabletop devices, computerized and automatic osmometers, and handheld osmometers do exist.[7] Vapor pressure osmometers measure the osmolality of a solution by measuring the voltage difference between two thermistors, one exposed to a sample solution and the other exposed to only the pure solvent corresponding to the sample solution; this allows for a relationship between voltage and solute concentration to be determined, and thus the osmolality of a sample.[8][9] 

On the other hand, membrane osmometers measure the flow of solvent (often water) from a pure solvent container across a semipermeable membrane into a solution containing a solute and the same solvent. The flow across the semipermeable membrane can be measured as the osmotic pressure of a sample and is related to the concentration of solute in the sample solution.[1][10] The semipermeable membrane in this device only allows the flow of solvent and blocks the flow of dissolved particles. These osmometers are limited in their application by the range of osmolality they can measure and the membrane material with which the semipermeable membrane can be made.

Alternative methods to osmometers can be used to measure osmolality, including electrical conductivity and specific gravity.[11] Specific gravity was utilized before osmometers were practical to measure the osmolality of urine, utilizing various instruments such as a refractometer (the measurement of the refraction of light through a fluid) or a hydrometer. The applicability and accuracy of methods besides osmometry to determine a sample's osmolality depend on the specific method and the sample.[8][9][10][11]

Specimen Requirements and Procedure

Specimen requirements vary between the various commercially-available osmometers and are included in the respective machine's instruction manual and laboratory protocols for specimen preparation. Acceptable specimens, in general, include serum, heparinized plasma, tears, sweat, and urine.[1][12] Some osmometers allow for the use of gel tubes to hold samples. Urine, in general, does not need to be collected with preservatives. It is important to centrifuge specimens prior to analysis to remove any particulate matter. Dilution is not required with many machines but may be needed with specific types of samples.[13][14]

The stability of different samples varies, though some general guidelines are as follows:

• Unspun serum/plasma (room temperature or refrigerated at 2 to 8 degrees Celsius): 3 hours
• Separated serum/plasma (room temperature or refrigerated at 2 to 8 degrees Celsius): 48 hours
• Urine (room temperature): 24 hours
• Urine (refrigerated or frozen): 7 days

The general procedure for osmometry starts with using a properly calibrated osmometer. A full calibration of osmometers is required at least every six months. It may also be necessary following a part replacement or service of the instrument or if the quality control (QC) reading is repeatedly out of range. For quality control, it is required that the laboratories run two controls at two different concentrations daily or with each batch of samples. Acceptable controls are generally within +/- 2 standard deviations from the mean. The institution's quality control policy and protocol and the instrument user manual should be followed to ensure accurate sample analysis.

Testing Procedures

The testing procedure for specific osmometers varies and is referenced in the user manual for each device. A general procedure outline can be found below.

General Osmometry Procedure

1. Enter or scan sample ID.
2. Load the sample to be tested into the osmometer. For freezing point depression osmometers, a small amount of sample is drawn in a sampler with a plunger. The sampler tip is inserted into the instrument's sample port, and the analysis begins. Loading the sample into the osmometer may start the analysis automatically. Other systems may require the lab technologist to press a button to start the process.
3. Record the sample osmolality.
4. Once the test is completed, withdraw the sampler from the osmometer and discard the sampler tip. Ensure the sampler and sample chamber are both cleaned appropriately.

Interfering Factors

There are a few limitations of Freezing point depression osmometers. Particulate matter can cause premature crystallization of samples; this can be avoided by centrifuging blood and urine samples or the filtration of urine samples. The presence of air bubbles in a sample may also result in the inadequate freezing of samples. Test samples that are allowed to evaporate, are non-aqueous in nature, or are highly viscous may not freeze well and yield an inaccurate result. High concentrations of substances, including ethanol, methanol, acetone, paraldehyde, trichloroethane, or propylene glycol, may not freeze reliably, leading to inaccurate results.

Finally, failure to clean the sample container after sample analysis can lead to the next osmometer measurement being skewed by the previous sample. Vapor pressure osmometers are limited by a lack of precision compared to freezing point depression osmometers. They cannot be used to measure the osmolality of solutions that contain volatile solutes.[1]

Results, Reporting, and Critical Findings

Certain sample types are more prone to inter-sample variability and may need two or more replicates. Volatile samples, for instance, may need to be performed in duplicate and agree within a certain range of mOsm. In these cases, the osmometer's specific protocol and user manual should be followed. The measurement range of osmometers varies depending on the instrument and sample being used; however, a common reportable range for freezing point depression osmometers is 0 to 2000 mOsm/kg. The normal reference range for serum is 275 to 295 mOsm/kg, and urine is 300 to 750 mOsm/kg, though these ranges may vary.[15][16] 

Clinical Significance

Serum Osmolality

Changes in serum osmolality can be useful in differentiating the causes of various electrolyte and acid-base balance disorders.[1][3] Serum osmolality is an important initial test used in the investigation of hyponatremia. Hyponatremia in the context of normal serum osmolality is a pseudohyponatremia. This is caused by the electrolyte exclusion effect when sodium is measured by the indirect ion-selective electrode (ISE) method in patients with increased lipids (hypertriglyceridemia) or total protein. Hyponatremia in the context of elevated serum osmolality (>295 mOsm/kg) is most commonly caused by elevated serum glucose, or it may also occur when osmotically active substances such as mannitol, radiographic contrast agent, and glycine (surgical irrigant solutions) are administered. Serum osmolality can be directly measured by osmometers, and it can also be calculated by using the measurements of osmotically active substances in the serum. Many formulas for the calculation of osmolality exist; however, the most commonly used and simple formula is the Smithline-Gardner formula presented below:

Osmolality (mOsm/kg) = 2 Na (mmol/L) + glucose (mg/dL)/18 + urea (mg/dL)/2.8[3][17] 

The difference between the calculated and measured osmolality is known as the osmol gap (also known as the osmolal or osmolar gap). A normal osmol gap is 2 or less, though according to some sources, it is greater than 10 mOsm/kg in the clinical environment.[1][18]

An elevated serum osmol gap can be found in the following clinical scenarios/disease states:

• Pseudohyponatremia: A low reported serum sodium concentration (less than 135 mEq/L) in the presence of normal measured serum osmolality. This can be the result of hyperlipidemia or hyperproteinemia, in which case these molecules make up an increased portion of the serum sample, resulting in a falsely decreased sodium reading. Based on the aforementioned Smithline-Gardner equation, the falsely decreased sodium concentration will decrease the calculated serum osmolality, resulting in an elevated osmol gap when compared to the normal range measured osmolality.[1][19]
• Exogenous substances: Ingestion of toxic alcohols such as ethanol, methanol, propylene glycol, and acetone can cause elevated osmol gap. The presence of an elevated osmol gap and suspicion of volatile alcohol poisoning often prompts further investigation by a volatile screen.[20] Additionally, osmotically active medications such as mannitol or glycerol are sometimes used to manipulate the osmol gap to drive fluid shifts within the body. One example of this is the use of mannitol to shift fluid from the brain into the blood to relieve increased intracranial pressure. The increased solute in the blood after mannitol ingestion results in an osmotic gradient moving fluid across the blood-brain barrier.[21] An important but often overlooked cause of exogenous substances elevating a patient's osmol gap is the ingestion of propylene glycol from administering medications that use this molecule as a vehicle, such as nitroglycerin, lorazepam, or etomidate.[18]
• Endogenous substances: Metabolic states such as diabetic ketoacidosis or alcoholic ketoacidosis increase the concentration of ketones in the bloodstream, increasing the osmolality within the blood and creating an osmol gap. Lactic acidosis can result in moderate lactic acidosis due to the accumulation of products from the breakdown of glycogen.[18] A hyperglycemic hyperosmolar state can also drive an increased osmol gap secondary to hyperglycemia.[22] Additionally, diseases that result in the retention of small molecules, such as renal failure, can also create a similar effect.[15] Finally, shock can result in the accumulation of cationic or neutral amino acids, also increasing the osmol gap.[18]

The value of change in the osmol gap can also give some insight into the cause of the elevation. For instance, renal failure and ketoacidosis often present with an osmol gap equal to or less than 15 to 20 mOsm/kg, while an osmol gap greater than 20 mOsm/kg often indicates alcohol or acetone accumulation. Lactic acidosis often results in an average osmol gap of approximately 11 mOsm/kg.[18]

Urine Osmolality

Urine osmolality is useful in the investigation of various electrolyte disturbances, particularly in the presence of hypo- or hypernatremia.[23][24] Hypernatremia and hyperosmolar plasma in the context of hypoosmolar urine (< 300 mOsm/kg) is suggestive of diabetes insipidus (DI). In this disease large volume of urine is excreted due to failure to produce an antidiuretic hormone (ADH) by the hypothalamus (central DI) or lack of response to ADH by the kidney (nephrogenic DI). Hypernatremia in the presence of hyperosmolar urine (> 600 mOsm/kg) is suggestive of extrarenal water loss (diarrhea, burn, or fever) or sodium overload by the administration of hypertonic saline or salt poisoning.[24]

In patients with low serum osmolality and clinically normal extracellular fluid volume, urine osmolality can distinguish between various etiologies. In these cases, urine osmolality >100 mOsm/kg is indicative of the syndrome of inappropriate antidiuretic hormone (SIADH), hypothyroidism, or glucocorticoid deficiency. On the other hand, a urine osmolality <100 mOsm/kg is seen in patients with primary polydipsia or Beer drinker's potomania.[23]

Stool Osmolality

Measuring stool osmolality can be helpful in the evaluation of patients with diarrhea to differentiate osmotic diarrhea from secretory diarrhea. The stool osmotic gap is defined as the difference between the measured osmolality and a calculated osmolality (2*([fecal Na+] + [fecal K+]).[25] A stool osmotic gap <50 mOsm/kg implies that the diarrhea is secretory. In patients with osmotic diarrhea, the stool osmotic gap is typically >75 mOsm/kg. It is essential to measure the osmolality within 30 minutes of collection of the stool sample, or the sample should be refrigerated until stool osmolality is measured because bacterial metabolism produces osmotically active substances that can alter the reading.[26] The stool osmolality <290 mOsm/kg can be the result of the non-accidental addition of water to a stool sample, and a stool osmolality >330 mOsm/kg without an elevated serum osmolality can imply that the measured sample was not stored appropriately, or it was contaminated with concentrated urine.[27]

Quality Control and Lab Safety

Accurate osmometry results require using a properly calibrated osmometer. More information on this is available in the specimen requirements and procedure section. The safe operation of an osmometer includes a variety of factors. The device must be plugged into an approved electrical outlet, as exposure to AC voltage could lead to electric shock, burns, or electrocution. Power cords should be adequately secured to ensure they do not create a walking or tripping hazard. The device should not be placed or operated on any wet, unstable, or non-leveled surfaces. Furthermore, osmometers should not be operated in an explosive atmosphere or areas with extreme or highly variable temperatures.

Enhancing Healthcare Team Outcomes

Determining serum and urine osmolality and identifying the difference between the calculated and measured osmolality (measurement of osmolar gap) are essential components in diagnosing and treating patients in various clinical scenarios. The modern osmometers in clinical laboratories are simple to operate and produce rapid and accurate results using a small volume of specimens. For better patient management, it is recommended that osmolality measurements are performed stat. This ensures better accuracy and minimum errors. Effective communication between all members of a patient's interprofessional care team, from laboratory technologists to nurses, physicians, pharmacists, and other health professionals, is necessary for patients to receive appropriate and timely medical care. A systematic process for stat measurement of osmolality is beneficial, as is a properly equipped and calibrated osmometer and fully qualified and trained laboratory technologists to run this instrument.[28][9][29]